1. Field of the Invention
The present invention relates generally to the field of breath sampling devices for alcohol monitoring systems. More specifically, the present invention discloses a breath alcohol sampling system that includes spirometric client identity confirmation.
2. Background of the Invention
Biometric identification is the process of recognizing or rejecting an unknown person as a particular member of a previously characterized set, based on biological measurements. The ideal biometric characterization is specific to the individual, difficult to counterfeit, robust to metabolic fluctuations, insensitive to external conditions, easily measured, and quickly processed.
Fingerprint, retinal, iris, and facial scans are well-known biometric identification techniques relying on image processing. Images are two-dimensional, requiring sophisticated and computationally intensive algorithms, the analysis of which is often complicated by random orientation and variable scaling. Voice recognition is an example of biometric identification amenable to time series analysis, an inherently simpler one-dimensional process.
The simplest biometric identifiers can be expressed as a single parameter, such as height or weight. Single parameter identifiers have been the only quantitative means of identification throughout most of history. The price of simplicity is the loss of specificity, and in the case of weight, the lack of constancy over time. Nevertheless, single-parameter biometrics remain effective identifying factors, as is obvious from their continued use.
Client identity confirmation (CIC) is the process of periodically verifying the identity of a particular individual. More precisely, the goal is to distinguish a characterized “client” among an open-ended set of similar but uncharacterized individuals. The objective is to ensure the primary biometric measurement (e.g., breath alcohol concentration) is not falsified by the client colluding with an impostor. CIC is somewhat simpler than identification, because it merely requires distinguishing the subject from all others rather than distinguishing every individual from every other. Typically, the service period is several months, short enough to be free of the confounding effects of aging.
Spirometry is a pulmonary function testing technique for measuring airflow and lung capacity, also known as lung volume. Various spirometric parameters, along with the flow-volume loop described in the next section, are promising for CIC because they vary widely among individuals, but are fairly stable from measurement to measurement for a specific individual over a typical service period, and resist counterfeiting. It is apt to compare spirometric parameters with the familiar biometric human height—they have similar specificities (ratio of population range to individual stability) and immunities to deception.
The spirogram is a plot of lung volume versus time during a maximal inhalation and exhalation, which can diagnose airway obstructions and constrictions, inadequate diaphragm function, or thoracic cage abnormalities. FIG. 1 is a schematic spirogram of a forced vital capacity test, consisting of a maximal inhalation followed by a forced exhalation. Inhalation is depicted with a dotted line, because the invention measures only exhaled breath. This spirogram plots lung volume versus time over one cycle of maximal inhalation and forced exhalation. Spirometry is a mature clinical diagnostic, and was standardized decades ago by the American Thoracic Society (ATS).
Among the several measures of lung volume, the forced vital capacity (FVC), defined as the difference between the volumes of maximum inhalation and exhalation, and the forced expiratory volume in the first second (FEV1) are particularly suit the invention. Because FVC measures the maximum air volume expellable in a single breath, it is physiologically impossible for the subject to overblow, so a measurement significantly greater than the baseline established during sensor “enrollment” indicates collusion with a cohort with more FVC than the subject. A measurement significantly lesser than the baseline indicates deception, involving either collusion with a cohort with less FVC than the subject, or the subject deliberately reserving exhalation to avoid a deep lung sample. FEV1, which is rather independent of FVC, may be the most reproducible flow parameter.
The time derivative of the spirogram gives the airflow versus time. The most prominent feature of this curve is the peak expiratory flow (PEF), which is correlated to but distinct from FEV1. The PEF's chief utility is that an operational shortfall relative to the enrollment baseline during operation indicates the subject is not maximally exhaling, possibly with deceptive intent.
The flow volume loop (FVL) is a plot of lung volume versus airflow, thus eliminating time as an explicit variable, while retaining implicit dynamical information. As the term “loop” implies, the FVL is cyclical or nearly so. The FVL encompasses all the spirometric parameters discussed above, therefore the shape of a client's FVL must be at least as specific as the spirometric parameter set. As the FVL may be the easiest representation of spirometric data to interpret and the most informative, it is incorporated into the example embodiment of the invention below.
FIG. 2 is a schematic FVL of a forced vital capacity test, with exhalation consisting of the positive-flow portion of the loop (solid line), proceeding counterclockwise from peak volume at time zero. The FVL plots airflow versus lung volume over one or more cycles of maximal inhalation and forced exhalation. Time has been eliminated as an explicit variable, but advances in the counterclockwise direction indicated by arrowheads. By convention, the time origin is placed at the lung capacity maximum. One can read the PEF and FVC directly from the FVL plot in FIG. 2. The exhaled volume can be found by integrating flow over time, and FEV1=V(0)−V(1).
Diagnosis is the chief clinical application of the spirogram and related plots. Consequently, the primary aim in the medical literature is to establish norms for spirometric parameters and FVLs, according to sex, age, height, and so on. The secondary aim is sometimes to identify an ailment according to the nature of its deviation from the norm.
Furthermore, clinicians are also concerned with repeatability, to best discern borderline abnormalities and therapeutic progress. The ATS has defined repeatability as the largest and median results of three maneuvers (recorded exhalations) must differ by no more than 0.2 liters, for both FVC and FEV1. Considering that a ballpark value for either parameter is two liters, the spirometry session is deemed unrepeatable if either ΔFVC is more than 10% of FVC, or ΔFEV1 is more than 10% of FEV1.
Repeatability appears readily achievable. In one study of 18,000 adult patients, only 5% of the patients were unable to match their highest FEV1 within 150 ml, and half matched their two largest FEV1's within 58 ml, or 3% of FEV1 (“Repeatability of Spirometry in 18,000 Adult Patients”, P. L. Enright et al., Am. J. Respir. Crit. Care Med. 169, pp. 235-238 (2004)). This result was irrespective of patient sex or age. Other groups have performed repeatably—a study of 852 children reported 87.9% achievement of ΔFVC less than 5% (“Forced expiratory manoeuvres in children: do they meet ATS and ERS criteria for spirometry?”, H. G. M. Arets et al., Eur. Respir. J. 18, pp. 655-660 (2001)). In a study of 7,101 sufferers of chronic pulmonary obstructive disease (COPD), approximately 86% met the criterion of less than 50 mL absolute and 10% relative, for either ΔFEV1 or ΔFVC (“Variability of Spirometry in Chronic Obstructive Pulmonary Disease”, L. B. Herpel et al., Am. J. Respir, Crit, Care Med. 173, pp. 1106-1113 (2006)). Other studies have reported good repeatability with children, the elderly and asthmatics.